Method and apparatus for drying compressed air

ABSTRACT

The heat of compression of compressed air heats the generator of an absorption refrigeration system to dry the air. An auxilliary heater for the generator supplies stand by heat when the compressor is not operating.

United States Patent 1 1 1111 3,739,594 Freese June 19, 1973 [54] METHODAND APPARATUS FOR DRYING 2,339,185 1/1944 NetteI 62/238 X COMPRESSED AIRI 654,577 7/I90O Abrams 62/233 X [76] Inventor: Charles E. Freese, 2916Homer A FOREIGN PATENTS OR APPLICATIONS Avenue, Erie. Pa. 165061,942,078 4/1971 Germany 62/476 Filed Jan 21 1972 453,513 II/I927Germany n 62/238 [21] APP! 219,657 Primary Examiner-William F. O'DeaAssistant Examiner-Peter D. Ferguson 52 11.5. CI. 62/93, 62/101, 62/238,Attorney-Ralph Hammar 62/33I, 62/331, 62/476 II;- Cl. ABSTRACT [58]Field of Search 62/101, 238,148,

62/331, 476, 93 The heat of compresslon of compressed a1r heats thegenerator of an absorption refrigeration system to dry [56] ReferencesCited the air. An auxilliary heater for the generator supplies UNITEDSTATES PATENTS stand by heat when the compressor is not operating.l,792,227 2/193I Kuhl 62/148 X 4 Claims, I Drawing Figure METHOD ANDAPPARATUS FOR DRYING COMPRESSED AIR This invention relates to a methodof removing moisture from compressed air by cooling with absorptionrefrigeration wherein the heat needed to generate the refrigeration isobtained from the compressed air itself.

In the drawing, the single FIGURE is a diagram of a preferred form ofcompressed air system.

In the drawing, 1 indicates the compressor and 2 the absorptionrefrigeration system of which the major units are the generator 3, thecondenser 4, the absorber 5 and the evaporator 6. Air at ambienttemperature enters the compressor intake 7 and is discharged through thecompressor outlet 8 at a temperature of about 350F due to the heat ofcompression. After passing through heat exchangers (to be described)associated with the absorption refrigeration system, the air in a driedstate and at near'ambient temperature is discharged through line 9 readyfor use in air operated tools and equipment. The heat of compression issufficient to operate the absorption refrigeration system and providethe cooling effect required to lower the dew point far enough so thatmoisture is not released in the compressed air line and in the airoperated tools and equipment.

The first heat exchanger 10 is between the inlet line 11 and thegenerator 3. The heat transferred from the incoming compressed air atabout 350F lowers the incoming air temperature to about 200F andcorrespondingly increases the temperature in the generator,

driving refrigerant (e.g. ammonia) out of solution into the condenser 4where it is cooled and condensed and discharged through a pressurereducing valve 12 into the evaporator 6.

The second heat exchanger 13 is between a continuation 14 of inlet line11 and air returning to line 9 after being cooled by the evaporator 6.The returning air at a temperature of about 40F is heated to about 150Fby the 200F air leaving the first heat exchanger 10. In the heatexchange process the temperature of air in line 14 is reduced to about100F. The second heat exchanger 13 may be replaced by other heatexchangers capable of reducing air temperature to near ambient, such asa conventional after cooler or a fan cooled air to air heat exchanger,either of which will reduce the 200F air temperature leaving the firstheat exchanger to near 100F. The second heat exchanger is needed toreduce the temperature of the air so that full advantage can be taken ofthe sub ambient cooling effect of the evaporator in removing moisturefrom the air by reducing the air temperature well below ambient. Themoisture content of the air is determined by its lowest temperature.

The third heat exchanger 15 is between the evaporator 6 ,andcontinuation 16 of line 14. This heat exchanger cools the air to about40F, a temperature far enough below ambient to cause enough moisture todrop out due to the lowered dew point so no moisture will be released inthe air operated tools and equipment. The moisture is caught in a trap17 and periodically released through a float valve l8. The refrigerantboiled off in the evaporator is recycled back through the absorber 5 tothe generator 3. The cooled air leaving heat exchanger 15 is returnedthrough heat exchanger 13 to line 9.

For intermittent operation, which is most common,

a stand by heater 19 is provided to keep the absorption system inreadiness for cooling. The heater 19 can be a gas burner fed from gasline 20 through a valve 21 controlled by the refrigerator temperaturecontrol 22. When the compressor is loaded, the burner flame can bereduced by partially or wholly closing the valve 21. When the compressoris unloaded, and no air is being compressed, the burner flame can beincreased to a value sufficient to maintain the absorption system instand by condition.

Starting with a heating medium at 350F, the commercially availableabsorption refrigerating systems required about 2 Btu input for eachlBtu of cooling. In the present system, there is nearly 3 Btu[(350-200)/(100-40)] for each Btu of cooling. There accordingly isusually enough heat in the compressed air itself to dry the air. If theheat of compression is not adequate, any deficiency can be supplied bythe gas burner 19. If the heat of compression is more than adequate, thesurplus cooling can be used for other purposes such as cooling thecylinder water jacket of the compressor 1.

Other heat exchangers may be added to reduce air temperature if desired.They may be inserted at the outgoing line or between heat exchanger 10and 13.

It is expected that other types of absorption refrigeration systems suchas those using lithium bromide may be used. The type used would dependon operating conditions and other considerations out of the scope ofthis invention.

What is claimed is:

l. The method of drying compressed air which comprises the steps of l.cooling a stream of compressed air containing heat of compression andunwanted moisture by passing it in heat exchange relation to thegenerator of an absorption refrigeration system,

2. further cooling the compressed air leaving step (1) by passing it inheat exchange relation to the compressed air leaving step 3), therebyreheating the compressed air leaving step (3),

3. further cooling the compressed air leaving step (2) by passing it inheat exchange relation to the evaporator of said absorptionrefrigeration system,

4. trapping moisture which drops out of the air as it is cooled.

2. The method of claim 1 in which stand by heat is supplied to thegenerator during periods when air is not being compressed so as tomaintain the absorption refrigeration system in readiness for drying.

3. Apparatus for drying a stream of compressed air containing heat ofcompression and unwanted moisture comprising an absorption refrigerationsystem including a generator, an evaporator and associated condensingand absorbing means, a first heat exchange means between said generatorand said stream of compressed air for heating the generator bytransferring the heat of compression to the generator, a secondheatexchange means for further cooling the air leaving the first heatexchange means comprising an air to air heat exchanger between the airleaving the first heat exchange means and the air leaving a third heatexchange means for further cooling the air leaving the firstheatexchange means by transferring heat to the air leaving the thirdheat exchange means, the third heat exchange means being between the airleaving the second heat exchange means and said evaporator for stillfurther cooling the air leaving the second heat exchange means input bytransferring heat to the evaporator, and means for for the generatorduring periods when air is not being trapping moisture which drops outof the air as it is compressed so as to maintain the absorptionrefrigeracooled. tion system in readiness for drying.

4. The apparatus of claim 3 having a stand by heater

1. The method of drying compressed air which comprises the steps of 1.cooling a stream of compressed air containing heat of compression andunwanted moisture by passing it in heat exchange relation to thegenerator of an absorption refrigeration system,
 2. further cooling thecompressed air leaving step (1) by passing it in heat exchange relationto the compressed air leaving step (3), thereby reheating the compressedair leaving step (3),
 3. further cooling the compressed air leaving step(2) by passing it in heat exchange relation to the evaporator of saidabsorption refrigeration system,
 4. trapping moisture which drops out ofthe air as it is cooled.
 2. further cooling the compressed air leavingstep (1) by passing it in heat exchange relation to the compressed airleaving step (3), thereby reheating the compressed air leaving step (3),2. The method of claim 1 in which stand by heat is supplied to thegenerator during periods when air is not being compressed so as tomaintain the absorption refrigeration system in readiness for drying. 3.Apparatus for drying a stream of compressed air containing heat ofcompression and unwanted moisture comprising an absorption refrigerationsystem including a generator, an evaporator and associated condensingand absorbing means, a first heat exchange means between said generatorand said stream of compressed air for heating the generator bytransferring the heat of compression to the generator, a second heatexchange means for further cooling the air leaving the first heatexchange means comprising an air to air heat exchanger between the airleaving the first heat exchange means and the air leaving a third heatexchange means for further cooling the air leaving the first heatexchange means by transferring heat to the air leaving the third heatexchange means, the third heat exchange means being between the airleaving the second heat exchange means and said evaporator for stillfurther cooling the air leaving the second heat exchange means bytransferring heat to the evaporator, and means for trapping moisturewhich drops out of the air as it is cooled.
 3. further cooling thecompressed air leaving step (2) by passing it in heat exchange relationto the evaporator of said absorption refrigeration system,
 4. trappingmoisture which drops out of the air as it is cooled.
 4. The apparatus ofclaim 3 having a stand by heater for the generator during periods whenair is not being compressed so as to maintain the absorptionrefrigeration system in readiness for drying.